1. Field of the Invention
The present invention relates to an ultrasonic technique that images the interior of a living body by transmitting and receiving ultrasound to and from the living body. In particular, the invention relates to an ultrasonic imaging technique that performs imaging with the use of a microbubble contrast agent.
2. Background Art
Ultrasonic imaging apparatuses that image the interior of a living body by transmitting and receiving pulsed ultrasound to and from the living body have been widely used for medical diagnoses.
Among imaging modalities, in the fields of X-rays and MRI in particular, contrast agents have previously been used for imaging a blood circulatory system and the like. The object of using such contrast agents is to obtain a contrast-enhanced image of the structure or distribution of a blood circulatory system by administering the contrast agents into the blood and thereby to diagnose diseases that are reflected by a blood circulatory system such as malignant tumors or infarctions with high accuracy.
In the meanwhile, contrast agents have not been widely used for ultrasonic diagnoses so far. These years, however, they have become to be widely used with the advent of contrast agents formulated by stabilizing fine bubbles (microbubbles) on the order of micrometers in size with some methods. The principle of microbubbles is as follows: microbubbles with a diameter of about one micrometer resonate with ultrasound with a frequency of several MHz, which is used for ultrasonic diagnoses, and thereby oscillating with large amplitude; consequently, ultrasound within such a frequency range is scattered well, increasing contrast sensitivity.
Microbubble ultrasound contrast agents are characterized by the strong non-linearity. This is due to the property of microbubbles that they expand in volume under negative pressure much more than they contract under positive pressure with the same amplitude. Accordingly, echo signals scattered from microbubbles include many second harmonic components that have a frequency two times that of a transmitted signal. V. L. Newhouse et al. reported a method of obtaining a Doppler signal of a blood flow that emphasizes soft tissue based on such second harmonic components, for the first time in 1992 (see Non-Patent Document 1, for example).
P. N. Burns et al. have proposed a pulse inversion method in which two times of transmission/reception are performed using transmission sound-pressure pulse waveforms whose polarities are inverted with respect to each other, and two echo signals obtained thereby are summed (see Patent Document 1, for example). By such summation, the fundamental components of the echo signals received from soft tissue whose motion can be disregarded will be cancelled out because a signal that is shifted in phase by 180° is added. Meanwhile, the second harmonic components will grow twice as large because a signal that is shifted in phase by 360° is added. Although the number of required transmissions increases double, it is in principle possible to eliminate the fundamental components from soft tissue without using a bandpass filter. Thus, second harmonic echo signals with excellent axial resolution can be obtained. As for a scatterer such as a microbubble contrast agent in a blood flow, of which changes occurring during the two times of transmission/reception cannot be disregarded, fundamental echo signals emitted from the scatterer cannot be completely cancelled out. However, the method of P. N. Burns is rather suited for the current objective of obtaining an echo signal that emphasizes a contrast agent relative to soft tissue.
Umemura reports in Non-Patent Document 2 a method of differentiating a contrast signal and a non-linear living-body signal by summing echo signals, which have been obtained as a result of performing three times of transmission/reception using pulses whose phases are shifted by 0°, 120°, and 240°. According to such a method, it is possible to cancel out second harmonics that have a constant phase relationship with the fundamental wave, concurrently with the fundamental wave. With such properties, it is possible to distinguish between second harmonics (e.g., second harmonics received from a contrast agent) whose phase does not have a constant relationship with a transmitted wave and second harmonics received from living-body tissue.
Bouakaz reports a method (see Non-Patent Document 3, for example) that includes transmitting and receiving a first chirp signal and transmitting and receiving a second chirp signal that is obtained by inverting the first chirp signal about the time axis, wherein a cross-correlation function (a first cross-correlation function) for a signal, which is obtained by inverting the first transmission signal on the time axis, and a signal received in response to the first transmission is determined, a cross-correlation function (a second cross-correlation function) for a signal, which is obtained by inverting the second transmission signal on the time axis, and a signal received in response to the second transmission is determined, and the difference between the first cross-correlation function and the second cross-correlation function is determined. When such a method is used, echo signals received from the respective microbubbles will differ from each other because the timing at which the frequency of the first chirp signal coincides with the resonance frequency of the microbubbles differs from that of the second chirp signal.
[Patent Document 1]U.S. Pat. No. 6,095,980[Non-Patent1992 IEEE Ultrasonics Symposium Proceedings,Document 1]pp. 1175-1177[Non-Patent2003 IEEE Ultrasonics Symposium Proceedings,Document 2]pp. 429-432[Non-Patent2006 IEEE Ultrasonics Symposium Proceedings,Document 3]pp. 224-227